No additional cracks were found during inspections of engines in the rest of the F-35 fleet or in any spare engines, Hawn said.

Matthew Bates, a spokesman for Pratt & Whitney, which supplies the engines for the planes, said the Pentagon’s F-35 program office had decided to lift a temporary suspension of flight operations after it conducted extensive tests on the affected engine part. Pratt & Whitney, a unit of United Technologies Corp, is sole supplier of engines to the $396 billion F-35, or Joint Strike Fighter. Lockheed Martin is the prime contractor for the radar-evading jet, the biggest weapons program in history.

The tests showed a crack in a turbine blade stemmed from the “unique operating environment” in flight tests rather than a design flaw, he said.

Bates said Pratt had been working around the clock with Pentagon officials to determine the cause of the crack in the engine blade.

“The team has determined that root cause is sufficiently understood for the F-35 to safely resume flight,” Bates said.

The engine in question had operated during testing at high temperatures more than four times longer than a typical F-35 flight, which led to a separation of the “grain boundary” of this particular blade, he said.

Current engines would not reach the same “hot time” for years, Pratt engineers have said, which would allow the Pentagon to impose incremental limits on engine use and monitor them for possible component replacement, one of the sources said.

“Basically this engine was run for an extraordinary amount of time at very high power in a short period of time,” said a source, who was not authorized to speak publicly.

The Pentagon announced the grounding of all F-35 warplanes last Friday after an inspection revealed a crack on a turbine blade in the jet engine of an F-35 being tested at Edwards Air Force Base in Kern County.

It was the second engine-related grounding in two months of the F-35 Joint Strike Fighter.

The Marine Corps version of the plane was grounded for nearly a month starting in mid-January because of a faulty hose in the engine.

In a related matter, Air Force Lt. Gen. Christopher Bogdan, who runs the F-35 program for the Pentagon, slammed Pratt and Lockheed during an air show in Australia on Wednesday, accusing the companies of trying to “squeeze every nickel” out of the United States government and failing to see the long-term benefits of the project.

“What I see Lockheed Martin and Pratt & Whitney doing today is behaving as if they are getting ready to sell me the very last F-35 and the very last engine and are trying to squeeze every nickel out of that last F-35 and that last engine,” Bogdan told reporters at the Australian International Airshow in southern Victoria state.

Bogdan was in Australia seeking to convince lawmakers and generals to stick to a plan to buy 100 of the jets, an exercise complicated by the second grounding of the plane this year and looming U.S. defense cuts.

“I want them both to start behaving like they want to be around for 40 years,” he added. “I want them to take on some of the risk of this program, I want them to invest in cost reductions, I want them to do the things that will build a better relationship. I’m not getting all that love yet.”

Bogdan’s tough remarks sent shock waves through the Pentagon and U.S. industry.

Lockheed said it was “singularly focused” on executing its contracts to develop, produce and sustain the new warplane, and insisted it was on track to finish development by 2017.

Pratt & Whitney issued a strongly worded response later Wednesday, saying it had invested heavily to cut the cost of the plane’s engine and was shouldering more risk than usual.